1. Field of the Invention
The present invention relates to a lithium battery comprising a positive electrode, a negative electrode and a nonaqueous electrolyte and more particularly, to a lithium battery adapted for dissolution and deposition of lithium metal in the negative electrode, the battery directed to a suppressed formation of lithium dendrite on the negative electrode and to a suppressed reaction between the lithium metal in the negative electrode and a solvent in the nonaqueous electrolyte in the lithium dissolution/deposition process associated with the charging and discharging of the battery.
2. Description of the Related Art
As a novel secondary battery of high power and high energy density, a lithium battery have gained widespread use. Such a battery employs the nonaqueous electrolyte and is capable of being charged and discharged through oxidation/reduction of lithium.
This lithium battery uses a variety of negative-electrode materials for forming the negative electrode.
Where lithium metal is employed as the negative-electrode material, the battery can achieve the highest theoretical capacity of 3.86 Ah/g. Accordingly, studies have been made on the use of lithium metal as the negative-electrode material.
However, if the lithium battery employing lithium metal as the negative-electrode material is subjected to repeated charging/discharging processes involving lithium-metal dissolution/deposition in the negative electrode, the negative electrode encounters a gradual growth of lithium dendrite thereon. The lithium dendrite will further grow through a separator to come into contact with the positive electrode. Furthermore, the dendrite growth leads to an increased contact area between the lithium metal of the negative electrode and the solvent of the nonaqueous electrolyte so that the lithium metal of the negative electrode is more likely to react the solvent of the nonaqueous electrolyte. As a result, the lithium battery is decreased in the charge/discharge efficiency and hence in the cycle performance.
More recently, Japanese Unexamined Patent Publication No.11-3713(1999) has proposed a lithium battery employing a negative electrode collector formed from a porous material, such as carbon, for suppression of the formation of lithium dendrite during charging and discharging of the lithium battery.
Unfortunately, the lithium battery with the negative electrode collector formed from the porous material, such as carbon, still suffers the drawback that the charge/discharge efficiency is decreased due to the reaction between the solvent or the like in the nonaqueous electrolyte and the lithium metal retained at the negative electrode collector or the porous material, such as carbon, forming the negative electrode collector.
A first object of the invention is to prevent the growth of lithium dendrite on the negative electrode during charging/discharging of the lithium battery adapted for the dissolution/deposition of lithium metal in the negative electrode.
A second object of the invention is to prevent the reaction between the lithium metal in the negative electrode and the solvent in the nonaqueous electrolyte in the lithium battery adapted for the dissolution/deposition of lithium metal in the negative electrode.
A third object of the invention is to enhance the charge/discharge efficiency and thence the cycle performance of the lithium battery adapted for the dissolution/deposition of lithium metal in the negative electrode.
In accordance with a first aspect of the invention, a lithium battery comprises a positive electrode, a negative electrode with a negative electrode collector retaining lithium metal, and a nonaqueous electrolyte, wherein an activated carbon with a minimum pore size in the range of 5 xc3x85 to 16 xc3x85 is used as the negative electrode collector.
In accordance with a second aspect of the invention, a lithium battery comprises a positive electrode, a negative electrode with a negative electrode collector retaining lithium metal, and a nonaqueous electrolyte, wherein an activated carbon with a specific surface area in the range of 960 m2/g to 2000 m2/g is used as the negative electrode collector.
With the minimum pore size of 5 xc3x85, the activated carbon used as the negative electrode collector has a specific surface area of about 960 m2/g. With the minimum pore size of 16 xc3x85, the activated carbon used as the negative electrode collector has a specific surface area of about 2000 m2/g.
If the activated carbon with the minimum pore size in the range of 5 xc3x85 to 16 xc3x85 and the specific surface area in the range of 960 m2/g to 2000 m2/g is used for forming the negative electrode collector of the negative electrode, as in the lithium batteries of the first and second aspects hereof, lithium ions with the solvent desorbed therefrom are inserted into the activated carbon pores so as to be deposited as lithium metal in the activated carbon pores when the lithium battery is charged for lithium metal retention at the negative electrode collector. Thus, the growth of lithium dendrite on the negative electrode is suppressed while the lithium metal deposited in the activated carbon pores is prevented from reacting the solvent in the nonaqueous electrolyte. Hence, the lithium battery is not degraded in the charge/discharge efficiency, accomplishing the improved cycle performance.
If the negative electrode collector is formed from the activated carbon with the minimum pore size of less than 5 xc3x85, lithium ions are not readily inserted in the activated carbon pores, resulting in the deposition of lithium metal on the surface of the activated carbon.
Consequently, the lithium dendrite is formed on the negative electrode or the lithium metal of the negative electrode is in contact with the solvent in the nonaqueous electrolyte, reacting the same. If, on the other hand, the negative electrode collector is formed from the activated carbon with the minimum pore size in excess of 16 xc3x85, solvated lithium ions are inserted into the activated carbon pores to be deposited as lithium metal. The resultant lithium metal is in contact with the solvent and the like of the nonaqueous electrolyte infiltrating into the activated carbon pores and the reaction therebetween occurs. Hence, both of the above cases entail the decreased charge/discharge efficiency and thence, the degraded cycle performance of the lithium battery.
The activated carbon for use in the negative electrode collector may take any form of grains, fibers and the like so long as the aforementioned properties are |presented. However, a fibrous activated carbon may be preferred from the standpoint of easy handling when used for forming the negative electrode collector.
In accordance with a third aspect of the invention, a lithium battery comprises a positive electrode, a negative electrode with a negative electrode collector retaining lithium metal, and a nonaqueous electrolyte, wherein a graphitized carbon is used as the negative electrode collector.
If the graphitized carbon is used as the negative electrode collector as in the lithium battery of the third aspect hereof, the lithium dendrite growth on the negative electrode is suppressed during charging of the lithium battery for lithium metal retention at the negative electrode collector. In addition, the negative electrode collector is prevented from reacting the solvent and the like in the nonaqueous electrolyte during charging. Thus, the lithium battery is improved in the charge/discharge efficiency.
Where graphitized carbon is used as the negative electrode collector, the higher the graphitization degree of the graphitized carbon, the greater the effect of suppressing the reaction between the negative electrode collector and the solvent and the like in the nonaqueous electrolyte during charging. The graphitized carbon having spacing d002 of lattice planes (002) in the range of 3.35 xc3x85 to 3.43 xc3x85 or more preferably of 3.35 xc3x85 to 3.36 xc3x85, or containing crystallite whose size along the c-axis length (Lc) is not less than 40 xc3x85 or more preferably of not less than 700 xc3x85, in particular, may be employed to achieve an even greater suppression of the reaction between the negative electrode-collector and the solvent and the like in the nonaqueous electrolyte during charging. Thus, the lithium battery is further improved in the charge/discharge efficiency.
It is noted that the lithium batteries of the first to third aspects hereof are characterized by the negative electrode collectors for use in the negative electrode and therefore, no particular restriction is posed on the positive electrode or the nonaqueous electrolyte of the lithium batteries.
The above lithium batteries may employ any of the known positive electrode materials conventionally used in the art. For example, metal oxides capable of absorbing and desorbing lithium ions are usable, including metal oxides containing at least one of manganese, cobalt, nickel, iron, vanadium, niobium and the like, and lithium-containing transition metal oxides such as lithium-containing LiCoO2, LiNiO2, LiMn2O2 and the like. Particularly if any one of the above lithium-containing transition metal oxides is used as the positive electrode material, the lithium in the transition metal oxide is retained by the negative electrode collector, negating the need for previously providing a lithium metal portion retained by the negative electrode collector. Therefore, the lithium battery may be fabricated without using lithium metal.
The above lithium batteries may employ, as the nonaqueous electrolyte, any of the known nonaqueous electrolytic solutions comprising a solvent and a solute dissolved therein, and of gel or solid state polymeric electrolytes.
The nonaqueous electrolyte may employ any of the known solvents commonly used in the art. Examples of a usable solvent include organic solvents such as ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, cyclopentanone, sulfolane, dimethylsulfolane, 3-methyl-1,3-oxazolidine-2-one, xcex3-butyrolactone, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, methyl propyl carbonate, butyl methyl carbonate, ethyl propyl carbonate, butyl ethyl carbonate, dipropyl carbonate, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyl tetrahydrofuran, 1,3-dioxolane, methyl acetate, ethyl acetate and the like. These solvents may be used either alone or in combination of two or more types.
Any of the known solutes commonly used in the art may be dissolved in the above solvent. Examples of a usable solute include lithium compounds such as lithium trifluoromethanesulfonate LiCF3SO3, lithium hexafluorophosphate LiPF6, lithium perchlorate LiClO4 lithium tetrafluoroborate LiBF4, lithium trifluoromethanesulfonimide LiN(CF3SO2)2 and the like.
Any of the known polymers commonly used in the art are usable as the gel or solid state polymeric electrolyte.
Examples of a usable polymer include polyethylene oxide, polypropylene oxide, crosslinked polyethylene glycol diacrylate, crosslinked polypropylene glycol diacrylate, crosslinked polyethylene glycol methylether acrylate, crosslinked polypropylene glycol methylether acrylate and the like.